Abstract

The distinctive Mg-smectite that is the predominant clay mineral in the Cretaceous/ Tertiary (K/T) boundary marl at Stevns Klint, Denmark (that is, the Fish Clay), which has been previously believed to have formed from impact-derived glass, is also present in the upper of two marls of probable early Danian age at Limhamn Quarry, Sweden, approximately 8 m above the K/T boundary. This Danian marl is devoid of anomalous iridium. We contend that this Mg-smectite is the result of alteration of a volcanic ash precursor, perhaps reworked, and that impact debris is a relatively minor component of the K/T boundary marls studied herein. K-Ar dates demonstrate that Caledonian detrital potassic phases (that is, illite) are present in the coarse clay fraction of the Fish Clay. The Fish Clay is thus composed predominantly of volcanic, detrital phases and lesser amounts of impact phases. In addition, we have discovered subhedral labradorite in the organic-rich black clay of layer III of the Fish Clay. Therefore, we suggest that pyroclastic basaltic volcanism was the predominant progenitor of the boundary clay unit and is responsible for its high Mg content.

Our results indicate that the main carrier of Ir in the K/T boundary marls is of submicron size and that Ir is mobile during diagenesis and/or weathering. In addition, our results show that Pt and Au are present in the upper Limhamn marl, and, for comparison, in the Permian Kupferschiefer Shale at amounts comparable to those of the K/T boundary. It is thus possible that some or much of the Pt and Au at the K/T boundary is derived from nonimpact sources similar to those that apparently contributed to the parts-per-billion concentrations of Pt and Au in the Permian Kupferschiefer Shale.

We cannot reject the impact hypothesis summarily, because shocked quartz with multiple planar sets is distinct to the K/T boundary, and this distinctive shocked quartz has not yet been identified in distal volcanic tephras or flows. If shocked quartz is of impact origin, then the collision of either a smaller asteroid, or a comet, is more consistent with the relatively small amount of impact-derived material in the Fish Clay. Alternatively, if shocked quartz is proven to be formed by explosive volcanism, then our evidence would permit explaining the K/T boundary events as entirely due to volcanism rather than impact.